↪ This law states that, " The ratio of mass of any two element which separately combine with the fixed mass of third element is same or simple whole number multiple ratio of mass of the same elements when combined themselves."
↪ The simple whole number ratio between the mass of combining element is obtained only when they combined to each other in the proportion of their equivalent weight. Because of this the law of reciprocal proportion is also called law of equivalent proportion.
Eg: In H2S, 32g of S combines with 2 gram of hydrogen.
In SO2, 32g of S combined with 32 gram of oxygen.
Then the ratio of mass of hydrogen in H and oxygen in SO2 is 2:32 = 1:16
Again in H2O ratio of mass of hydrogen and oxygen is 2:16 = 1:8
Therefore (1:16)2 = 1:8 which is equal to 2nd ratio.